Backend Development
PHP Tutorial
Simple analysis of the running process of the PHP program, analysis of the php process_PHP tutorial
Simple analysis of the running process of the PHP program, analysis of the php process_PHP tutorial
简单解析PHP程序的运行流程，解析php流程
一直想了解Web编程的技术。PHP是进行Web编程重要的一种语言，书上总是说，PHP是用于服务器端的编程语言。但是，实在不能理解它是怎么用于服务器端编程，如何被运行。也看过一些PHP的教程，大多是介绍语法，却没有给出前面的问题解答。最近东拼西凑的看了一些东西，终于对PHP如何运行有了一些了解。
HTTP协议与PHP脚本的触发
HTTP是一种基于Request/Response的协议，是支持Web运行的协议基础。HTTP的客户端发送Request
到服务器端，服务器端返回Response。Request中包含客户端需要访问的页面的文件名。服务器端返回该文件名指向的网页。如果没有使用PHP、JavaScript等，HTTP协议传输只能是静态的HTML文件。也就是，HTML文件不会受到用户行为的影响，内容一直保持不变。
如果要实现动态网页，就需要使用PHP或JavaScript。PHP是用于服务器端的编程语言，JavaScript是多用于客户端的编程语言。
PHP代码是在服务器端被执行的。用户访问一个包含PHP代码的网页时，发送Request到服务器，其中包含网页的文件名。服务器收到Request后，找到文件名指向的文件，发现其中嵌有PHP代码，会调用PHP解释器处理该文件，然后将处理后的结果整理到Response，发送到客户端。PHP代码可以与服务器端的数据库或其他资源进行交互，或者根据用户的操作产生不同的页面。
因此，PHP脚本的触发是在服务器收到客户端的Request。收到一个Request后，服务器触发一个PHP脚本；处理完脚本后，返回结果到客户端，等待下一个Request。当收到下一个Request后，服务器触发另一个（或同一个）PHP脚本。两次PHP脚本的运行是相互独立的，第二次脚本的运行几乎不受前一次脚本运行的影响。
JavaScript代码一般是在客户端被执行的，即被浏览器所处理。客户端从服务器端获取的是JavaScript代码，而不是代码被执行后的结果，然后调用解释器执行该代码。
PHP的代码块
PHP代码是可以嵌入到HTML文件中的，经常可以在HTML文件中看到散落在各处的PHP代码块。我也一直困惑在同一个HMTL文件中，不同的PHP代码块之间是怎样的关系。原来PHP是会忽略两个PHP代码块之间HTML代码的。
<head>
<title>Test PHP Multiple Blocks</title>
</head>
<body>
<p>This is HTML code.</p>
<?php
$var = 1;
echo "<p>This is PHP code block 1. <br/> \$var=$var </p>";
?>
<p>This is HTML code too.</p>
<?php
$var += 1;
echo "<p>This is PHP code block 2. <br/> \$var=$var</p>"
?>
</body>
代码中有两个PHP代码块，它们被HTML代码隔开了。第1个PHP代码块声明了1个变量$var；第2个代码块引用$var，进行改变了它的值。用PHP执行上述代码，得到的输出如下。
<head> <title>Test PHP Multiple Blocks</title> </head> <body> <p>This is HTML code.</p> <p>This is PHP code block 1. <br/> $var=1 </p> <p>This is HTML code too.</p> <p>This is PHP code block 2. <br/> $var=2</p> </body>
第2个代码块是可以引用$var的。虽然被HTML代码隔开了，但是两段代码的执行完全忽略了HTML部分。PHP的输出与下面没有被HTML隔开的PHP代码是一样的，前者只是在PHP输出结果之间插入了相应的HTML代码。
<?php
$var = 1;
echo "<p>This is PHP code block 1. <br/> \$var=$var </p>";
$var += 1;
echo "<p>This is PHP code block 2. <br/> \$var=$var</p>"
?>
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